(1) Field of the Invention
This invention relates generally to methods of reducing interference among cellular telephone signals, and among aircraft cellular telephone signals and ground cellular telephone signals. The apparatus disclosed includes, for aircraft, horizontally-polarized directional radio-frequency antennae comprising patch antennae or an array of virtual loop antenna elements.
(2) Description of the Related Art
Existing cellular telephone systems use both directional and non-directional antennae. Directional radio-frequency antennae are well known because most types of antennae exhibit some directional characteristics. Directional antennae are used in radar, direction finding, microwave transmissions, short-wave broadcasts, and in other applications, while in commercial radio broadcasting the signal is generally broadcast in all directions.
Cellular telephone systems for ground stations comprise numerous mobile cellular telephone stations which transmit and receive ultrahigh-frequency signals to and from a network of cellular telephone base stations, herein referred to as ground cellular base stations or as ground base stations. The ground base stations are connected to the switched telephone network. Timing and control of cellular calls is performed by a mobile telephone controller.
Ground base stations include antennae mounted on towers, buildings, or other elevated locations. The areas covered by the various base stations necessarily overlap somewhat in order to produce full coverage of a service area. Determination of which base station will control a call to or from a particular mobile station is on the basis of signal strength; the base station at which the signal received from the mobile station is strongest will be assigned to control the call.
The cellular system in North America uses a two-frequency cellular telephone channel for each call, one frequency for transmitting and one for receiving. A total of 832 such channels are available for cellular telephone use, between the frequencies of 824 MHz to 849 MHz, and 869 MHz to 893 MHz.
Antennae on ground mobile cellular stations such as automobiles, trucks, and boats are vertically polarized, and antennae on base stations are likewise vertically polarized to provide more efficient coupling between the antennae. As will be seen below, a different polarization between base and mobile stations has a marked effect on the effectiveness of transmissions between the two stations.
A particular problem is encountered when attempting to use a cellular telephone from an aircraft. Because of the elevated position of a mobile cellular telephone station aboard an aircraft, its signal is broadcast over a wide area of the earth's surface, where it is received by several ground cellular base stations. A high-flying aircraft is at a substantially equal distance from several ground cellular base stations, and a signal transmitted from the aircraft is received at approximately the same strength by these base stations. The similarity in the received signal strength causes difficulty for the ground system in determining which ground base station should control the call. This problem, along with the resulting interference, has led to the current prohibition of the use of airborne mobile cellular telephone stations.
With regard to aircraft antennae and directional antennae, applicant is aware of the following patents which are disclosed because they may be considered material by the examiner:
______________________________________ U.S. Pat. No. Issue Date Applicant ______________________________________ 5,121,128 June 9, 1992 van Lith de Jude, et al. 4,595,928 June 17, 1986 Wingard 4,352,200 Sep. 28, 1982 Oxman 4,325,121 Apr. 13, 1982 Ghose 4,117,491 Sep. 26, 1978 Hanna et al. 3,641,580 Feb. 8, 1972 Monser ______________________________________